1. Field of the Invention
The present invention relates to a method for the combined removal and destruction of nitrate ions, and, in particular, to a method in which the removal and destruction of nitrate ions from aqueous solutions is carried out by electrochemical ion exchange.
2. The Prior Art
Nitrate is an increasingly important species in water, particularly in cases where the water is extracted for human consumption. High nitrate levels, particularly in excess of 50 ppm, in drinking water have been thought to be responsible for the incidence of "Blue Baby Syndrome", and more speculatively, for increased incidence of stomach cancer. As a result, interest is focusing on finding methods of removing nitrate from water, particularly for the treatment of potable water sources.
The leading technology at the present time for the treatment of nitrate is ion exchange. In this process, nitrate ions are removed from the water on an anion exchange resin, which is periodically regenerated with appropriate chemicals, most notably brine (sodium chloride). The removal of nitrate from the water by this process is highly efficient, but the collected nitrate finishes up in the waste solution which results from the brine regeneration process. The disposal of the waste solution may incur significant waste disposal costs or regulatory complications.
Conventional nitrate ion exchange treatment has benefitted by the advent of nitrate selective ion exchange resins (which have special functional groups e.g. quaternary ethyl ammonium). An example of such a resin is PUROLITE (Trademark) A 520, a macroporous styrene-divinylbenzene strong base anion resin.
Other technologies are also available for nitrate treatment, such as bio-denitrification, but these have not found such favor as the ion exchange treatment.
Nitrate ions are capable of being destroyed by electrochemical processes, because nitrate can be converted by oxidative and reductive processes into harmless constituents such as water, nitrogen and oxygen as described in U.S. Pat. No. 3,542,657. It is also known that anions migrate through anion-selective membranes under the influence of an electric current. The process of migrating ions through membranes into a separate compartment in which electrode reactions are taking place is termed "electrodialysis". However, the disadvantage of electrodialysis is that the percentage of unwanted ions removed on any pass through the cell is usually quite low, which leads to the requirement for recirculation, complexity of equipment and poor economics.
We have now developed a method for the combined removal and destruction of nitrate ions which combines the benefits of ion exchange and electrodialysis.